A period of seven days beginning with Sunday or Monday.
A period of five days beginning with Monday.
A subdivision of the month into longer periods of work days punctuated by shorter weekend periods of days for markets, rest, or religious observation such as a sabbath.
A 4-day week consists of Monday, Tuesday, Wednesday, and Thursday.
(following a named day) A date seven days after (sometimes before) the specified day.
I'll see you Thursday week. [a week on Thursday, i.e. Thursday after next]
The wedding is tomorrow week. [a week tomorrow, i.e. in eight days' time]